Largest structure on the planet built by living organisms, Australia’s coral rampart hosts a carnival of sea life.

Sometimes
at its outer edge and sometimes closer, the beam from the flashlight
kept reflecting off big cat eyes. They glimmered pale silver, with
pupils darker than the darkness through which they glided. But cats
don’t patrol 40 feet (12 meters) deep in the Coral Sea. These
were sharks. It was hard to tell what kind they were, but some of the
shadowy bodies looked a lot longer than mine.

I breathed up my
scuba tank’s air sooner than planned and had to surface far from
the boat. Then I was swimming through black swells toward the
ship’s distant light as though mired in one of those dreams where
you need to move faster but can’t. I promised myself that it
would be a while before my next night dive on Australia’s Great
Barrier Reef. Yet within days I was beneath a full moon and 50 feet (15
meters) of water looking at more cat eyes.

These belonged to
an epaulette shark, small and lovely and speckled, lithe as an eel as
it curled round a coral pillar. Two lionfish with fins like flared wing
feathers cruised upside down beneath a coral table as though that were
the seafloor. Above them a pinnacle of coral twisted nearly to the
surface, lit from behind by the ship’s lamps and the moon.

Silhouetted,
each shelf, frond, curlicue, and fan emphasized the eerie
configurations that develop in the near absence of gravity. Drifting
weightless beside them seemed like sightseeing on another planet. But
the moment I thought that, I realized that I had it wrong. This scene
was the very essence of our home planet, which is, after all, ocean
blue. A single coral wall holds a broader representation of earthly
life—species from more phyla, or major groups—than an
entire continent does. It seems otherworldly only to those of us born
above tide line.

Coral reefs form when colonies of tropical
marine plants and animals with limestone skeletons rise atop earlier
generations. They fashion the most visually diverse natural
environments a human can experience, and the Great Barrier Reef is the
world’s single largest coral domain. With the broad, shallow
continental shelf of tropical northeastern Australia providing an ideal
pedestal for growth, this coral complex reaches as far as 160 miles
(260 kilometers) offshore and more than 1,250 miles (2,000 kilometers)
from north to south. The Great Barrier Reef covers 135,000 square miles
(350,000 square kilometers), an expanse greater than Poland.

To
explore what amounts to an offshore nation, photographer David Doubilet
and I roamed 4,000 miles (6,400 kilometers) on dive boats. We spent so
many hours submerged that I began to think land looked weird. The day I
found the same sort of remora fish that clings to sharks and manta rays
hitching a ride on my leg, I wondered if it might not be time to dry
out.

Although the name suggests a continuous strip, the Great
Barrier Reef is actually a commonwealth of at least 2,800 reefs. Only
some are true barrier reefs—breakwaters rising near the edge of
the continental shelf. In the calmer seas behind that cordon, more
reefs appear as irregular circles and crescents known as platform
reefs. Smaller formations, called patch reefs, are scattered throughout
shallow areas.

Fringing reefs grow outward from the
mainland’s shores, but more often they are found surrounding the
region’s 618 continental, or high, islands, which were mountains
and hills along Australia’s Ice Age coast before the glaciers
melted and raised sea levels; the Aborigines’ legends of ancient
generations walking out to those islands are true. In addition there
are about 300 low islands, or cays, formed atop coral shoals from reef
sediments. As seabird droppings glue the grains together and colonizing
plants build soil, some of those desert isles transform into shadowy
woodlands, while storms pound others back into shifting piles of sand.

By
acting as a buffer against heavy seas, the reef-and-island complex
makes possible neighboring sea grass beds and coastal mangrove forests.
Those in turn trap sediments, store nutrients, and serve as nurseries
for a number of reef residents. Now add the soft sea bottoms between
reefs plus submarine hillocks made entirely of Halimeda, a calcium-hardened green algae.

Put
all these habitats together with clear azure waters flowing from the
Coral Sea and brown, soil-laden waters washing off the continent. Mix
with currents, daily tides, and seasonal weather patterns. What you
have is the formula for the Great Barrier Reef ecosystem and lifetimes
of discovery.

The better we came to know this offshore nation,
the more it resembled a long chain of provinces. Four major sections
are widely recognized: Mackay/Capricorn in the south, where the water
first warms enough to encourage coral growth, Central, Cairns, and Far
Northern, which is the most remote and, being closest to the Equator,
hosts the lushest array of life.

Our first destination was the
Eastern Fields, a rarely visited atoll that lies outside the Far
Northern section near the Gulf of Papua, 200 miles (320 kilometers)
east of Australia’s Cape York Peninsula and approaching New
Guinea. This pristine marine wilderness represents a part of the
Pacific that seeded the continental shelf with coral some 20 million
years ago, after Australia drifted north into the tropics.

While
I strapped on an air tank, Duncan Johnstone, one of the professional
divers helping to crew our boat, offered advice. The waters, he said,
would be very clear, with visibility at 150 feet (45 meters) or more,
and very shacky, “shack” being how Aussies say
“shark.” What kind? “Whalers,” another name for
members of the requiem shark family. Great whites are rare in tropical
waters. Another big boy is not: the tiger shark, a whaler that grows to
20 feet (6 meters). But Johnstone was referring to the more common
five- to ten-foot-long (1.5- to three-meter-long) bronze whalers, gray
reef sharks, and silvertips.

“They hunt in small
packs,” he continued, “and they like to come right up for a
look, give you a bit of a squeeze. Just back toward the reef if you
can. Don’t go popping to the surface.”

Descending
past green coral that looked like sunken organ pipes, I got squeezed by
a pack right away. The whalers moved on, and I leveled out a hundred
feet (30 meters) down on the lip of a sheer drop. I hovered awhile,
then kicked out into nothing but blueness. Though my depth gauge gave a
constant reading, I couldn’t shake the perception of falling.
Southern bluefin tuna cruised under my feet. Schooling unicornfish and
trevallies, or jacks, swirled past like currents made visible. Far
bigger shapes bulked where the blue turned to gloom.

Feeling
more and more like a hapless mote of plankton, I turned back and
drifted up balcony-like tiers of coral toward the clerestory light far
above through chromis, sergeants, triggerfish, surgeonfish, and
clouds of other fish feeding on everything from algae to shrimps to the
corals that housed them. By the time I neared the anchor line, I knew
the main challenge ahead wasn’t going to be sharks. It was going
to be how to make sense of the poly¬chrome dazzle of creation that
is a reef.

Heaving aboard after a dive, I would head straight
for the identification books, thinking: Surely I can pick out the
rainbow fish with pink stripes radiating from its eyes. Aha, it’s
a wrasse. But there must be a hundred wrasses here—and at least a
dozen with that eye pattern. So is it the tailspot, threespot,
checkerboard, pinkbelly, moon, or sunset wrasse?

With new fish
species found in the Great Barrier area every year, the total is
approaching 2,000. It will likely keep rising. So will the estimates of
4,000 mollusks and at least 350 hard, or reef-building, corals.
Researchers counted more than 250 types of shrimps on the reefs just
around Heron Island, near the Great Barrier’s southern end. One
volleyball-size coral chunk there yielded 1,441 worms from 103 species.

Perhaps for want of enough names to go around, many of the fish
I swam with had labels borrowed from land animals: lizardfish, batfish,
hogfish, and that double steal, the foxface rabbitfish. Getting better
at recognizing species only made me more curious to know why there were
so many kinds in the first place. What drives this extravagance of
color and form? And how is such biological wealth even possible when
the clear, blue quality of tropical seas reflects a scarcity of
nutrients and plankton, the base of the food pyramid?

The
northern reefs form a nearly continuous ribbon for about 400 miles (640
kilometers). Near the Olinda Entrance, a narrow break in the bulwark, I
kayaked across the wave-beaten reef flat behind a coral slope, then
dropped overboard with snorkel gear. Dark shapes shot across the bottom
ten feet (three meters) below. I tried to follow before I realized that
the water was so transparent that I was chasing the shadows of brown
boobies flying overhead, carrying fish to the fluffy white chicks
waiting on a nearby cay. When I looked down again, I noticed some
seaweed tumbling in the current. Then the lacy brown clump hung a left,
circled a piece of coral rubble, and darted after a speck of food.

Pretty
good for algae, or rather for a juvenile rockmover wrasse that looks
and acts like drifting debris, right down to rolling from side to side,
presumably to fool predators. Another juvenile has the same flip-flop
swimming style, and one is thought to mimic a foul-tasting flatworm for
the same purpose. The fact that the adult fish may have different
patterns of fins, color, and behavior was my first clue to the puzzle
of the diversity around me.

Near the Portlock Reefs I
couldn’t figure out the dots moving behind a crack in a coral
ridge. My diving partner did, seeing the toothy end of a six-foot-long
(1.8-meter-long) black-spotted moray eel emerge to inspect my head from
behind. By then I was distracted by a gold-and-white juvenile bicolor
parrotfish. As adults, females are reddish brown and males are
jade-and-pink. Thus the animal comes in three distinct models. Other
fish produce half a dozen—a stronger clue as to how scores of
species around a coral knoll can look like hundreds.

A broad
spectrum of reef fish switch sex as they age or as changes in the
social environment trigger hormones that promote male characteristics
at the expense of female traits or vice versa. When the male fairy
basslet attending a harem disappears, the dominant female may start
acting like a male within hours and become one physically within a few
days.

Temporary color changes add to the complexities of
sorting out who’s who on the Great Barrier. I watched a brown
trumpetfish swim behind a slow-moving star puffer and match its gray
tones while using the big blowfish for concealment to sneak up on prey.

I lay on the deck and was just dozing off when it came to me.
Not enlightenment, but another fish—splat on my eye. No trouble
identifying this one: a flying fish that was soaring away from a
pursuer on long, winglike pectoral fins. Tiny squid and baitfish were
always jetting onto the stern steps. When Johnstone was filleting a
coral trout there for dinner, a bronze whaler surged up onto the
transom and thrashed around, trying its damnedest to help carve the
meat. Boobies, noddies, and terns perched and pooped all over the bow.
We seemed to be joining the ecosystem. No other boat had come into
sight for hundreds of miles. Perfect. Just us and the sea.

Part
of the pleasure of these unspoiled surroundings came from knowing that
Australia was managing them to stay that way. In 1975 virtually the
whole offshore nation was declared Great Barrier Reef Marine Park, one
of the world’s first national marine sanctuaries and still the
biggest. Intended mainly to prevent oil drilling and mining on the
reef, the park remains open to many other uses. Commercial fishing,
sportfishing, spearfishing, and the collecting of aquarium fish and
shells are regulated, however, and some segments are set aside as
no-take zones, research zones, or special reserves for troubled species
such as dugongs, marine mammals closely related to manatees.

Of
the globe’s seven species of sea turtles, six are found in the
Great Barrier region. The least imperiled is the flatback, which keeps
to Australian waters. Female green turtles travel 1,600 miles (2,575
kilometers) from places like Indonesia and New Caledonia to lay their
eggs in spots such as Raine Island, a northern cay. From the boat we
watched thousands transform the beach nightly with flailing flippers
and flying sand, scattering the seabirds that also come to nest.

Many
of the Great Barrier islands are overseen by the Queensland Parks and
Wildlife Service, which does much of the day-to-day work of supervising
the marine park as well. Raine Island, where Aborigines once came in
outrigger canoes to gather food, is now off-limits to most visitors to
protect the nesting wildlife, but we were free to swim along the coral
slopes it rests upon.

Turtles would pass so close we could
make out marks from shark teeth on their shells and chunks missing from
limbs. More of the 200- to 400-pound (90- to 180-kilogram) reptiles
rested until nightfall in caves on the reef wall. I never peered into
one of those dark nooks without also seeing the big eyes of
cardinalfish, squirrelfish, or other nocturnal species looking
back—yet another clue to the area’s extraordinary
biological wealth.

Coral reefs are riddled with caverns and
crevices—a microtopography that greatly multiplies both the total
surface area and the variety of niches available. This is where the
night shift spends the day, the day shift hides at night, and an
assortment of sea squirts, soft corals, and sponges, which lack rigid
skeletons, makes a full-time home. Tighter crannies take the place of
shells, spines, or foul-tasting chemicals as protection for other
creatures. Many, including some algae and sponges that look too soft
for the job, bore tunnels of their own.

Continuing southward
through the Far Northern section, we finally drew near enough to land
to see Cape Weymouth on the horizon, then reconnoitered underwater for
hours by Ferguson Reef. I sank down to rest on a sand patch, discovered
I was maybe a dozen feet (3.7 meters) from a whitetip reef shark doing
the same thing, and finger-walked over to a wider sand slope. The shark
never budged, but within a minute the barren-looking habitat had scores
of garden eels growing out of it, emerging from burrows as the shock of
my arrival wore off.

Bumphead parrotfish seemed awfully
fond of the place as well. Largest of the region’s 29 parrotfish
species, a bumphead can weigh more than a hundred pounds (45 kilograms)
and devour five tons (five metric tons) of coral a year. After gouging
out chunks with teeth fused into a beak, it chews them with a second
set of powerful jaws deep in the throat. As the fish defecated, a
curtain of coral slurry descended upon me, and I tried to keep in mind
the fact that many a perfect coral sand beach comes from parrotfish
dung.

Stay down long enough and even tropical seas drain your
body heat. To warm up, all I had to do was rise toward the surface,
stoked to bathtub temperature by the sun. When I floated over sand at
this upper level, the brightness made my eyes ache. The driving force
behind the abundance of reef life was becoming hard to miss: limitless
solar power. Just as on land, the marine food chain rests upon the
ability of plants to convert solar energy to food and building
materials. The secret to the fecundity of the Great Barrier is that so
many of the plants dwell inside animals.

Mistaken for colorful
plants themselves at one time, corals are actually carnivores related
to anemones and jellyfish. Like them, corals use tentacles with
stinging cells to snare microscopic prey. Corals can also catch food on
their mucous coating and absorb nutrients directly through their
epidermis. Even so, as much as 90 percent of their nourishment comes
from golden brown algae they host in their tissues at a density of
millions per square inch (millions per square centimeter). The
corals’ enzymes cause the algae to leak carbohydrates. In return
the algae get nitrogen from the corals’ waste material, along
with a home.

About 90 percent of giant clams’ food comes
from the same symbiotic algae. The clams grow these microscopic plants
beneath translucent panels, essentially farming them inside a fleshy
greenhouse. A surprising number of other animals nurture internal algae
too, from sponges to thin-skinned flatworms.

Partnerships play
a role almost anywhere you look on a reef. The largest marine fish
family is that of gobies, with more than 2,000 species worldwide. Many
of the known tropical species share burrows with shrimps, which keep
the holes clean while the gobies act as sentinels, warning of danger.
Not that the laws of tooth and maw have been repealed. During one dive
I saw a diagonal-banded sweetlips open wide to have parasites removed
from its mouth and gills by what looked like a cleaner wrasse. Instead
a fang-toothed blenny, an accomplished mimic of the cleaner wrasse, bit
off a hunk of the sweetlips’s inner cheek and skedaddled. Close
by, a big barracuda yawned to accommodate a true cleaner
wrasse—then snapped its teeth shut and gulped the thing.

Roughly
175 miles (280 kilometers) south of Ferguson Reef we crossed into the
Cairns section, where the Great Barrier generally runs nearest the
mainland, and stepped onto Lizard Island after three weeks at sea. The
ground seemed to sway so much I could hardly stand. Still, the prospect
of walking for more than the 72-foot (22-meter) length of our boat was
so exhilarating that I staggered all the way to Cook’s Look, the
island’s granite summit, 1,178 feet (359 meters) above sea level.

In 1770, during a voyage to find the rumored continent of Terra
Australis, Capt. James Cook of England scrambled up to search for a way
out of what he had begun to view as an endless barricade of reefs. The
great navigator had already holed the hull on the coral once. By 1900
the Great Barrier had claimed 1,200 vessels. We had made a few edgy
excursions ourselves through portions still marked
“Unsurveyed.”

During my visit Cook’s Look
was wrapped in glowing clouds. Torresian imperial pigeons called from
eucalyptus trees, and four-foot (1.2-meter) monitor lizards rustled in
the leaves underneath. The place spoke less of the European era than of
Dreamtime, the mystical age envisioned by Aborigines who once held
initiation rites for young men atop the peak.

Hiking on to the
Lizard Island Research Station, I tracked down Bob Podolsky, a marine
biologist visiting from the University of North Carolina at Chapel
Hill. He had come to study brittle stars, so named because their long
arms break off easily. While a detached arm coils and squirms,
distracting whatever grabbed it, the rest of the brittle star can
escape, soon to regrow the missing limb.

Podolsky focused a
microscope on a larva hatched from a minuscule brittle star egg. An
eater of algae, this nearly invisible plankton particle looked nothing
like the adult—or anything else, except maybe avant-garde jewelry
or an interstellar probe. I was remembering why my first childhood
glimpse of the animated cosmos within a water droplet hooked me on
biology forever when Podolsky said, “An adult brittle star puts
out more than a million larvae in its lifetime and yet will replace
itself with just a single offspring if it’s lucky. So the chance
of a larva surviving is literally one in a million.”

We
often say life is miraculous. Here was a statistic to back that up. To
reach maturity, each brittle star has to be the very fittest—and
luckiest—of the fit. The same holds true for many other organisms
in the coral realm, including corals themselves. Repeat such
one-in-a-million success stories every generation over millions of
years, and an amazing array of forms with spectacular adaptations
becomes not just possible but probable.

Three days later we
jumped in on Hastings Reef. Within easy reach of the port city of
Cairns, it turned into the busiest spot we had yet seen as arriving
boats discharged divers and snorkelers. The locale had a worn look that
came from a lot of flippered feet kicking the reef and hands grabbing
for a closer look. But it was also under assault by crown-of-thorns,
the huge, spiked, poison-tipped sea stars that dine on live coral.

As
I hovered by pajama cardinalfish—think chubby minnows in clown
outfits—a diver came by and thrust a long spear into one of the
stars. Periodic outbreaks of crown-of-thorns can strip reefs of color
and life. While scientists debate whether these are natural events,
like wildfire, or the result of human activities, such as the
overharvest of fish that eat juvenile crown-of-thorns, tour operators
hire people to remove adult stars daily from popular sites.

There
are only 3.5 million residents in the entire state of Queensland, but
the growing city of Cairns receives nearly 800,000 international
visitors yearly. Most come for a look at the Great Barrier. That the
sumptuous reefs in crystalline water shown on travel posters lie 25
miles (40 kilometers) or more offshore often comes as a surprise.
Boarding one of the high-speed catamarans that carry as many as 300
passengers there at a time, I found crew members stationed up front
with determined smiles and rubber gloves, ready to deal with
seasickness bags.

Once we reached Agincourt Reefs, the craft
docked at a huge, stable platform anchored by cables. Some groups
headed out on guided diving or snorkeling tours. I opted for the
ten-minute helicopter overview of the turquoise shoals. Afterward I sat
beside a German grandmother to motor past coral walls in a glass-sided
semisubmarine, then cooled down in the roped-off snorkeling area.
Looking at the paddlers around me, I realized that one way or another,
folk of every age and ability were getting to experience a world long
beyond the reach of most travelers.

After the Cairns area the
most visited part of the Great Barrier is the Whitsunday Group, islands
located in the Central section, where reefs are more widely scattered
than those farther north. The archipelago is a favorite of both foreign
travelers and Australians on holiday, especially sailing enthusiasts.
Since it sprawls close to the mainland, the waters aren’t very
clear. Yet when I submerged 30 feet (9 meters) off Hook Island, I found
a thriving mix of hard corals. Among them was an even richer assortment
of soft corals, swallow-tailed sea slugs with electric colors, and
scores of fish new to me.

I never did identify the little
yellow one that kept scooting up the loose wet suit sleeve of my diving
companion, Amanda Parr. She and a boatful of locals were replacing
marker buoys encrusted with marine growth. Such floats guide boaters to
mooring buoys put in place to relieve anchor damage to the
islands’ reefs.

Alongside Parr, a dive instructor, were
Tony Fontes, who trains dive instructors, and Elmer Ten Haren, who
oversees dive operations for a tour company. These people make their
living in the water. Here they were getting wet again on their day off
and working for free. When I asked why, Ten Haren answered simply,
“Because it needs to be done.”

Neither the
Queensland Parks department nor the Great Barrier Reef Marine Park
Authority has the manpower to keep on top of all the chores called for
along the vast reserve they are charged with managing.
“That’s where we come in,” said Fontes, co-founder of
a volunteer group that calls itself the Order of Underwater Coral
Heroes. “We use our diving expertise to maintain buoys, clean
junk off the reef, help with research projects, and educate the public.
We didn’t choose the name to sound noble. We liked the acronym
O.U.C.H. It lets us tell people that’s how coral feels when you
drag an anchor across it.” Just the same, I added genuine heroes
to the list of life-forms I had encountered along the reef.

That
list, growing longer hourly, led me to James Cook University in
Townsville, the main coastal city along the Great Barrier’s
Central section. I wanted a professional opinion as to why reef life
was such a parade of forms and hues. Why 20 look-alike butterflyfish
species instead of, say, two or three?

David Bellwood, a
leading marine ecologist, replied, “The answer is that we really
don’t know. But I can tell you that a lot of the diversity within
groups of tropical fish is a matter of historical accident. It may be
that as sea levels fell during the ice ages, ocean basins became
isolated, and their populations evolved along separate lines, which we
now regard as separate species. Later, when sea levels rose again, many
migrated to Australia and took up residence side by side.”

And
the communities continue to flourish today. “The best management
decision Australia ever made was to put its reef far offshore and along
the northern end of the continent, which hardly has any people,”
Bellwood said jokingly. “We have some overfishing, but this is
one of the few countries that has yet to make its big mistakes with
coral ecosystems.”

On the other hand, the coral realm is
not immune to the changes taking place in ecosystems on land. Cane
fields, other croplands, and development along Queensland’s
coastal plain have replaced many seaside wetlands, the natural filters
for fresh water coming from the continent. Coupled with deforestation,
overgrazing by livestock, and runoff from towns, farms, and industries
upstream, this sends more sediments and nutrients flowing out toward
the Great Barrier. The total has quadrupled since colonial times.
Corals can persist in surprisingly murky water as long as tides and
currents periodically sweep the sediments off. It’s the nutrients
that wreck a reef. Anything beyond moderate levels of nitrogen hurts
growth and reproduction in corals while fertilizing free-living algae
that can smother their neighbors.

Drilling down into coral
reveals distinct bands whose thickness is a measure of annual growth,
rather like the rings in a tree trunk. “Our samples show that
some reefs have stopped growing lately,” Jon Brodie, a
water-quality expert with the Marine Park Authority, told me. “We
can tie this directly to higher nitrogen levels in the water coming out
of rivers. But it’s important to point out that when we say the
Great Barrier Reef is at risk, we’re only talking about portions
of the inner reef so far. More remote areas and the outer reef as a
whole are still in good shape.”

Experts worry about an
apparent rise in the frequency of bleaching, a condition whereby corals
lose their symbiotic algae and turn white, most often when the water
gets unusually hot. Prolonged bleaching kills corals. Yet as Terry
Done, a senior research scientist at the Austra¬lian Institute of
Marine Science, pointed out, disturbance and renewal are common in
coral ecosystems and can even add to their diversity.

Reefs
proceed from infancy to maturity, sometimes followed by senility, when
erosion outstrips growth. At every stage corals are subject to periodic
bleaching and crown-of-thorns outbreaks. Not to mention hurricanes; a
typical reef on the Great Barrier gets hit every 20 to 50 years.
“Before you make pronouncements about the health of any
reef,” Done concluded, “you have to know its age and where
it stands in recovery from the most recent disturbance. We need to get
away from the notion that reefs everywhere at all times should look
like the perfect undersea garden in a travel brochure.”

A
week later I was exploring a reef that, like the garden of biblical
Eden, had a problem with snakes. After cruising steadily northeast from
the port of Gladstone, we were a hundred miles offshore in the Swain
Reefs complex of the Mackay/Capricorn section, the Great
Barrier’s widest belt of reefs. Sea snakes were everywhere,
wriggling in and out of coral branches after small fish and eels.
Though their venom is deadlier than a cobra’s, these marine
serpents aren’t bad tempered as a rule, just unnervingly curious.

A five-foot-long (1.5-meter-long) olive sea snake followed my
son, Russell, all the way around a reef, seemingly fascinated by his
blue flippers, nosing them whenever he paused. Higher up I found David
Doubilet staring through his camera viewfinder at a poisonous
stonefish, oblivious to the fact that he had a far more toxic sea snake
about to insinuate itself under one armpit and a shark checking out his
other side.

We all needed thick rubber suits because the water
here was several degrees lower than in the Far Northern section. The
reefs’ coral cover wasn’t as dense as before, and there
were fewer fish species, though the place was still a Mardi Gras of
fins and scales. I was trying to tell several kinds of angelfish apart
when a light absorbing hulk the size of a minibus showed up. It was a
700-pound (300-kilogram) Queensland grouper. That afternoon still wider
manta rays and eagle rays came flying through clouds of plankton in the
tidal current.

A mother and baby bottlenose dolphin arrived
toward evening for a long visit around the boat. Farther off more
dolphins arced from a silken sea into a papaya-colored sky. But the
calm within this shallow labyrinth of reefs was misleading. Crossing
open water the next evening, the ship was taking ten-foot-high
(three-meter-high) waves abeam and shuddering with each blow while the
wind blew 40 knots in the thickening darkness and I made deals with
God.

The lighthouse marking North Reef came into view long
after midnight. “About bloody time,” Capt. Norm Joseph
growled as we made for the protected side. I had tried to analyze reefs
in terms of geology, ecology, and management. At that moment I was
merely grateful beyond words that reefs existed.

The next
night I was lulled to sleep by the mewling of wedge-tailed shearwaters
on southerly Heron Island, where seabirds gather by the tens of
thousands each night during the nesting season. Our own long journey
was nearly over.

After crossing the island to a research
station run by Queensland University, I met Ove Hoegh-Guldberg, whose
research team may have made an intriguing discovery. “At shallow
depths sunlight here can actually be toxic,” he began.
“Corals have special pigments to absorb the ultraviolet rays, and
their symbiotic algae hide in the shade beneath bundles of these
pigments for protection. Conversely, at depths where little sunlight
penetrates, the algae nestle right inside the coral’s pigment
bundles. Then, as the pigments re-radiate the light energy they have
gathered, the algae can use it for photosynthesis.” This
re-radiation causes corals to fluoresce a phenomenon that Doubilet had
photographed before anyone fully understood how or why it occurred.

Beach
walking away from the research station at low tide, I came upon adults
bent over, scooping sand like kids making castles. They turned out to
be students led by Adrian Jones, a University of Queensland marine
ecologist. “I look at these flats as big green leaves,” he
said. “All sorts of microscopic algae and photosynthetic bacteria
turn out to be living among the sand grains, and our measurements show
that they put out as much oxygen as an equal area of tropical rain
forest canopy. They also produce complex sugars that bind sediments and
help keep the whole reef structure together.”

A
lifelong diver, Doubilet was always reminding me how the invention of
scuba suddenly opened humanity’s eyes to a realm that had
remained a mystery since time immemorial. I pressed my toes into the
warm, damp sand. How often had I strolled over bright tropical beaches,
never imagining that they too were a scarcely known frontier suffused
with one-in-a-million creatures?

Everything about the Great
Barrier Reef felt like a revelation. To truly understand the
rock-solid, organic fantasia that is a coral reef, we may need to
assemble the most sophisticated knowledge, technology, and theorems
available. Then again, maybe we will step back with a nod and say: This
is what life on Earth does, given warm water, sunlight, and time.

Tour this extraordinary aquatic world with photographer David Doubilet and meet the exotic creatures that thrive there.

Photographer David Doubilet describes one of the planet’s “great pulses of life.”

The one that got away from our coverage of Australia’s Great Barrier Reef is this month’s Final Edit.

The
Australian government has done a remarkable job protecting the Great
Barrier Reef. What successful conservation efforts are you seeing in
your area? Tell us about them.

In More to Explore the National Geographic magazine team shares some of its best sources and other information. Special thanks to the Research Division.

When it comes to diversity, coral reefs have just about every other
terrain beat. The same may hold for coral terminology. Coral and reef
terms have specific meanings. To find out what they mean, read on.

A coral is a colony of many individual coral animals called polyps
that are derived from replication of an original polyp. A coral reef is
made of “squillions” of coral colonies plus other limestone
depositing organisms, growing on and among the skeletons of their
predecessors, and the sands and silts derived from them. On the Great
Barrier Reef, depending on where it grows, a reef can be either a
ribbon reef (on the continental shelf edge), a platform reef (on the
shelf), or a fringing reef (along the continental islands and
mainland).

Great Barrier Reef Marine Park Authority
www.gbrmpa.gov.au
The official website of the Great Barrier Reef Marine Park provides
information about visiting the park and important conservation issues
concerning the reef.

Australian Institute of Marine Science
www.aims.gov.auThis organization, founded by the Australian government “to
generate the knowledge needed for the sustainable use and protection of
the marine environment,” hosts a site that includes guides on
coral bleaching, crown-of-thorns starfish outbreaks, pollution, and
dangerous marine animals.

CRC Reef Research Centre
www.reef.crc.org.auThe center, established by the Australian government, provides
scientific information, education, and training to enhance reef-based
industry and park management. Check out the section about the effects
of fishing on the reef.

Fishbase
www.fishbase.orgWant to know about a fish, almost any fish? Fishbase, a
comprehensive online database of the Earth’s fishes, includes
more than 25,000 species. Listings include photos, physical
descriptions, geographic distribution, environment, and more.

Quicksilver Tours
www.quicksilver-cruises.com/home.htm One of many companies that offer daytrips to the Great Barrier
Reef, Quicksilver’s catamaran, semisubmarine, and helicopter
rides were enjoyed by author Doug Chadwick.